There are a number of different positioning systems in existence today based on various principles including ultrasound and radio waves. These systems all have both advantages and disadvantages. The system according to the invention employs identification tags which are placed on the objects that have to be monitored. The identification tags contain both ultrasonic receivers and radio transmitters.
Systems based on radio waves, for example in one of the unlicensed ISM bands (Industrial, Scientific, Medical), for example 2.4 GHz for WLAN or Bluetooth™ plug-in wireless network card in PCs have the following advantages: they have a high data transfer capacity, they can tolerate a lot of movement (Doppler shift) and have a long range. The disadvantages of such a system are that the long range makes it necessary to use 3 or more base stations in order to obtain a position.
Location systems based on ultrasound have the following advantages: they have a short range, the waves do not pass through walls, i.e. they are easy to position at room level and the detectors are cheap. The disadvantages are that these systems have a low transfer capacity and cannot tolerate much movement.
The invention outlined herein combines the best features of radio and ultrasound-based systems. An identification tag according to the invention is equipped with both an ultrasonic receiver and a radio transmitter. The ultrasonic signals it receives are transmitted synchronously from a plurality of transmitter units. The ultrasonic receiver in the identification tag is connected with a calculating unit which calculates the arrival time of received ultrasonic signals. This information together with the identification tag's ID code etc. are transmitted in the form of radio waves to a central processing unit which calculates the identification tag's position and presents it to a user of the system.
By equipping an identification tag with a radio receiver in addition to a radio transmitter, the ultrasonic receiver in the identification tag can be activated only if ultrasonic signals are to be transmitted in the room or the area in which it is located.
The system may advantageously be employed in areas which are traditionally used for only radio-based systems or only ultrasound-based systems. By using ultrasound for calling up a single or several tags, the area in which tags respond will be restricted, while the use of radio waves for transferring the ID code and timing gives the identification tag a high data transfer capacity and makes it insensitive to Doppler shift.